Washington -LRB- CNN -RRB- -- The National Transportation Safety Board has concluded a deadly accident involving a bus driver who fell asleep while driving on a Virginia interstate last year was caused in part by `` acute sleep loss '' as well as inadequate oversight by the bus company and the government .

Four people died on May 31 , 2011 , when a motorcoach operated by Sky Express Inc. , with 59 people on board bound for New York , drifted off Interstate 95 near Richmond , Virginia , around 5 a.m. . The bus struck a cable barrier , rotated clockwise , then overturned . The roof was crushed . In addition to the four deaths , 49 other passengers were injured .

The driver , Kin Yiu Cheung , was slightly injured and refused medical treatment , safety officials said .

At a board meeting Tuesday , the safety board said that a 13-month investigation found the driver failed to maintain control of the vehicle `` because of fatigue resulting from acute sleep loss , poor sleep quality , and ... the failure of Sky Express Inc. management to follow adequate safety practices and to exercise safety oversight of the driver . ''

`` This crash never should have happened , '' said the board 's chairman , Deborah A.P. Hersman . `` It was entirely preventable . Those travelers were failed at three levels : by the driver , the operator and the regulator . ''

The safety board ruled out alcohol , drugs , cell phone use , weather , mechanical defects , speed and highway design as playing any role in the accident .

The report said the bus company 's failure to `` exercise even minimal oversight of its drivers ' rest and sleep activities enabled the drivers to drive while dangerously fatigued . ''

An examination of the driver 's work schedule , sleep times and cell phone use revealed that his opportunity for sleep in the 72 hours prior to the crash was limited , the safety board said in a release .

The board said another factor contributing to the accident was the Federal Motor Carrier Safety Administration 's lack of adequate oversight of North Carolina-based Sky Express , `` which allowed the company to continue operations despite known safety issues . ''

`` Protecting the traveling public by strengthening bus and truck safety is FMCSA 's number one priority , '' the federal organization whose primary mission is to prevent commercial motor-vehicle-related fatalities , said in a statement sent to CNN . `` FMCSA closed the loophole that allowed 10-day extensions for passenger bus companies undergoing safety compliance reviews . It is also why our agency shut down 54 unsafe bus companies in 2011 . ''

The report cited `` the lack of a comprehensive occupant protection system , including systems for providing passenger restraint and for ensuring sufficient roof strength . ''

In addition to determining the cause of the accident , federal safety officials recommended expanded research on window glazing requirements to protect occupants from ejection as well as developing ways to assess effectiveness of fatigue management plans to improve alertness and limit driving mistakes .

Sky Express shut down shortly after the accident .

Cheung was charged with four counts of involuntary manslaughter and reckless driving .

Motorcoaches transport 750 million passengers each year , according to the National Transportation Safety Board .

CNN 's Alicia Tarancon contributed to this report .

@highlight

The driver of the tour bus fell asleep , leading to the accident in which four passengers died

@highlight

The NTSB blames the driver , the bus company and the federal oversight agency

@highlight

`` This crash never should have happened , '' says the board 's chairman